Intravascular Ultrasound (IVUS) has become an important interventional diagnostic procedure for imaging atherosclerosis and other vessel diseases and defects. In the procedure, an IVUS catheter is threaded over a guidewire into a blood vessel of interest, and images are acquired of the atherosclerotic plaque and surrounding area using ultrasonic echoes. This information is much more descriptive than the traditional standard of angiography, which only shows an image of the blood flowing through the vessel lumen. Some of the key applications of IVUS include: determining a correct diameter and length of a stent to choose for dilating an arterial stenosis, verifying that a post-stenting diameter and luminal cross-section area are adequate, verifying that a stent is well apposed against a vessel wall to minimize thrombosis and optimize drug delivery (in the case of a drug eluting stent) and identifying an exact location of side-branch vessels. In addition, new techniques such as virtual histology (RF signal-based tissue characterization) show promise of aiding identification of vulnerable plaque (i.e., plaque which is prone to rupture and lead to onset of a heart attack).
There are generally two standard types of IVUS catheters: mechanical/rotational IVUS catheters and solid state catheters. In a rotational IVUS catheter, a single transducer consisting of a piezoelectric crystal is rotated at approximately 1800 revolutions per minute while the element is excited by a signal. This excitation causes the element to vibrate at a frequency dependant upon its thickness, which depending on the dimensions and characteristics of the transducer this can be from around 9 to 45 MHz. The single element transducer of the rotational IVUS catheter can be made very thin and therefore able to vibrate at relatively high frequencies, thus achieving a relatively high resolution, this is of particular value in the near field (close to the outside diameter of the catheter sheath). In addition, this type of catheter configuration facilitates the use of relatively high excitation voltages, increasing the signal to noise ratio.
In rotational IVUS catheters, the transducer subassembly is situated in a preformed metallic structure called a transducer housing that is attached via adhesive or weld to an end of the drive cable. The metallic transducer housing is often formed with stainless steel. A backing material is formed from a loaded liquid two-part epoxy that is applied to the back side of the transducer subassembly when the transducer subassembly is inserted into the transducer housing. The backing material is then cured to couple the transducer subassembly to the preformed metallic housing.
The preformed metallic housing can negatively impact image quality because it is constructed of a strongly reflective and resonant material. In addition, the transducer subassembly and transducer housing are usually very small, which makes reliable and consistent manufacture within small tolerances difficult. Secure and precise placement of the transducer subassembly within the preformed transducer housing can also be difficult.